In recent years, the removal of nitrogen and phosphorous from treated wastewater has become increasingly important because of the eutropification of natural water courses. In basic terms nitrogen removal is accomplished by converting ammonia contained in the mixed liquor stream to nitrites and nitrates, in the presence of oxygen and known as an aerobic nitrifying stage. Ammonia conversion to nitrite is carried out by microbes known as Nitrosomonas, while the conversion of nitrite to nitrate is accomplished by Nitrobacters.
Nitrate conversion to nitrogen gas occurs in an anoxic denitrifying stage that takes place in a suspended growth environment and is devoid of dissolved oxygen. Nitrogen, carbon dioxide and water is produced, with the gas being vented from the system.
Nitrification rates can be optimized by regulating interdependent waste stream parameters such as temperature, dissolved oxygen levels (D.O.), pH, solids retention time (SRT), ammonia concentration and BOD/TKN ratio (Total Kjeldahl Nitrogen, or TKN, is organic nitrogen plus the nitrogen from ammonia and ammonium). Higher temperatures and higher dissolved oxygen levels tend to promote increased nitrification rates, as does pH levels in 7.0 to 8.0 range. Sludge retention times of from 31/2 to 5 days dramatically increase nitrification efficiency, after which time efficiencies tend to remain constant.
Increases in ammonia concentration increases the nitrification rate but only to a maximum level attainable after which further ammonia concentration increases do less to increase the rate of nitrification. Rates have also been shown to be maximized at BOD/TKN ratios of less than 1.0.
Physical/Chemical phosphorous removal as can be achieved by the addition of lime, alum or iron salts. Biological phosphorous removal requires an anaerobic suspended growth zone at the start of the system, and a sludge fermentation tank to supply volatile fatty acids (VFA's) for the energy needs of the phosphorous ingesting organisms (Acinetobacters).
Autotrophic organisms are those that utilize energy from inorganic material and include the nitrifiers Nitrosomonas and Nitrobacters. Heterotrophs utilize organic energy sources and include the aerobic BOD removers and the Acinetobacter biological phosphorous removers (Bio-P organisms).
Refractory treatment and polishing stages may be added to the process, downstream of the final clarification stage. In many waste streams, the majority of organic compounds (80%-90%) are easily biodegraded. The remaining fraction biodegrade more slowly and are termed "refractory" compounds. Prior art biological nutrient removal designs incorporate a single sludge and a single clarifier, for example, U.S. Pat. No. 3,964,998 to Barnard, but in that case the overall oxidation rate of the system has to be reduced to satisfy the slowest compound to oxidize.
Biological nutrient removal (BNR) systems can take various process configurations. One such embodiment is the five stage Modified Bardenpho.TM. process, which is based upon U.S. Pat. No. 3,964,998 to Barnard. It provides anaerobic, anoxic and aerobic stages for removal of phosphorous, nitrogen and organic carbon. More than 24 Bardenpho.TM. treatment plants are operational, with most using the five stage process as opposed to the previously designed four stage process. Most of these facilities require supplemental chemical addition to meet effluent phosphorous limits of less than 1.0 mg/L. Plants using this process employ various aeration methods, tank configurations, pumping equipment and sludge handling methods. WEF Manual of Practice No. 8, "Design of Municipal Wastewater Treatment Plants", Vol. 2, 1991.
The specific purpose of each of the bioreactor zones of the modified Bardenpho process is as follows:
Anaerobic Zone A--selector zone to allow Acinetobacteria (known as Bio-P organisms) to internally store organic carbon derived from pre-fermented sludge (volatile fatty acids, or VFA's) for later use as an energy source in the aeration zone, where the Bio-P bacteria commence to take up phosphorous. There is no nitrate or dissolved oxygen in this zone. PA1 First Anoxic Zone B.sub.1 --reactor in which the nitrate present in the recycle flow from the aerobic zone is biochemically reduced to nitrogen gas (denitrification) in the presence of sufficient organic carbon to ensure rapid reaction rates. PA1 First Aerobic Zone C.sub.1 --organic carbon (BOD) is oxidized to carbon dioxide, ammonia nitrogen is oxidized to nitrate (nitrification) and the Bio-P organisms utilize the carbon that was stored in the anaerobic zone to take in large amounts of phosphate and store it internally. The phosphate is subsequently extracted from the system by wasting the sludge in which it is contained. PA1 Second Anoxic Zone B.sub.2 --reactor in which the nitrate not recycled to the first anoxic zone is converted to nitrogen gas (denitrification) but at slower rates due to lower levels of remaining organic carbon. PA1 Second Aerobic Zone C.sub.2 --reactor to which air is added to prevent significant continuing nitrate conversion to nitrogen gas (denitrification) in the final clarifier D, which would hinder solids settlement. PA1 Aeration Control: Colder influent temperatures require higher dissolved oxygen levels to encourage nitrification, but over-aeration results in the discharge of excess D.O. in the mixed liquor recycle line from the aerobic Zone C.sub.1 to the anoxic Zone B.sub.1, which will then require more carbon to maintain effective nitrogen dissolution rates. Since available carbon levels may be limited, insufficient denitrification may result, as well as impaired phosphorous removal. PA1 Nitrification Control: Nitrification is controlled by varying the D.O. levels in the aerated nitrifying zone (the higher the D.O., the higher the nitrifying rate) or by varying the solids retention time (SRT) in this zone. SRT is controlled by varying the volume of mixed liquor wasted from the nitrifying tank. SRT should be reduced for higher temperature influents and increased for lower temperature flow. Nitrifying organisms are very sensitive to toxins, which may inhibit nitrification. Mixed liquor wasting should be commenced if this condition is evident. PA1 Denitrification Control: The source of the organic carbon required for denitrification is the influent wastewater. If nitrate levels are too high in the effluent stream, it is indicative of insufficient carbon levels, or too much dissolved oxygen is being recycled to the anoxic zone. Carbon levels may be increased by adding more volatile fatty acids to the anaerobic zone. Under normal operating conditions ammonia in the effluent should be less than 1 mg/L, and the nitrates should be less than 2 mg/L. Over aeration may increase nitrates while underaeration may increase ammonia. PA1 Sludge Fermentation Control: Upstream sludge fermentation tanks thicken the sludge and generate VFA's required in the anaerobic zone. Primary sludge is retained in the tanks to allow acid fermentation, but retention time is limited to prevent methane production which is detrimental to biological phosphorous removal. Leslie, P. J., "Westbank Wastewater Treatment Plant--A Case History", Western Canada Water and Wastewater Association Biological Nutrient Removal Seminar, November 1993. PA1 (a) treating said wastewater under anoxic, denitrifying conditions with denitrifying bacteria to reduce the concentration of nitrate ion and produce nitrogen gas and a denitrified liquor; PA1 (b) treating said denitrified liquor in an aerobic vertical shaft bioreactor with an oxygen-containing gas to effect BOD removal by the bioxidation of organic compounds in said denitrified liquor and produce carbon dioxide off-gas and a shaft bioreactor effluent liquor; clarifying a first portion of said shaft bioreactor effluent liquor to provide a first clarified liquor and a second portion of said shaft bioreactor effluent liquor to provide a second clarified liquor; PA1 (c) treating said first clarified liquor under aerobic, nitrifying conditions with nitrifying bacteria, an oxygen-containing gas and said off-gas to oxidize ammonium ion to nitrate ion and provide a first nitrified liquor; PA1 (d) recycling by adding said first nitrified liquor to said wastewater under step (a); PA1 (e) treating said second clarified liquor under aerobic, nitrifying conditions with nitrifying bacteria, an oxygen-containing gas to oxidize ammonium ion to nitrate ion and provide a second nitrified liquor; PA1 (f) removing said second nitrified liquor as plant effluent. PA1 (g) treating raw wastewater influent under anaerobic fermentation conditions with volatile fatty acid-forming bacteria to produce a volatile fatty acid-containing liquor; PA1 treating said volatile fatty acid-containing liquor with phosphate-fixing bacteria to provide a phosphate-fixed liquor; and treating said phosphate-fixed liquor under step (a).
By optionally wasting some solids from the first aerobic Zone C.sub.1, phosphate that was taken up by the Bio-P organisms is removed from the system and may be put to beneficial use, such as soil additives.
The presence of sufficient low molecular weight organic carbon compounds entering the anaerobic zone allows the use of a smaller zone, and ensures a more uniform effluent concentration of phosphate. This may be achieved by fermentation of primary sludge with the subsequent supernatant added to the anaerobic zone as a source of VFA's for the Bio-P organisms. The second anoxic zone and the second anoxic zone is correct, and if sufficient low molecular weight organic carbon is available to the anaerobic zone. Oldham, W. K., "Biological Nutrient Removal from Wastewater--The Canadian Experience", The Canadian Civil Engineer, Vol. 10, No. 9, November 1993.
The control parameters include the following:
Unfortunately, the Modified Bardenpho process requires an overly large, capital intensive treatment plant having significant operating expenses to minimize operational difficulties. Accordingly, there is a need for an efficacious biological nutrient removal system that requires less operational control and capital cost.